Data communication is presently conducted over AC power lines within homes using the HomePlug 1.0 specification, which uses the well-known carrier sense multiple access with collision avoidance (CSMA/CA) technique for medium sharing. Using this mechanism, a station senses the medium to determine if the medium is busy. If the medium is determined to be idle, the transmission may proceed. If the medium is busy, the transmission should defer until the end of current transmission. After the end of current transmission, stations go through a priority resolution process in the priority resolution slots. The result of the priority resolution process is that stations with the highest priority in the network will contend in the contention resolution slots while other stations defer from accessing the medium. Proper operation of the HomePlug 1.0 protocol requires that all station properly determine the start of the priority resolution period. This is achieved by using the broadcast information contained in the frames, which the HomePlug 1.0 specification refers to as the MAC protocol data units (MPDUs).
Payload is exchanged between HomePlug 1.0 stations using what the specification calls a Long MPDU. A Long MPDU consists of a Start of Frame (SOF) delimiter, Payload, End of Frame Gap (EFG) and End of Frame (EOF) delimiter. A long MPDU can support eight different payload lengths. The SOF delimiter indicates whether a response is expected, the frame length (FL) and Contention Control (CC) Information. The EOF delimiter indicates whether a response is expected, Contention Control (CC) and Channel Access priority (CAP) information. HomePlug 1.0 stations use a Response delimiter to transmit MAC level acknowledgments. The Response delimiter indicates the type of response (ACK, NACK or FAIL), Contention Control (CC) and Channel Access priority (CAP) information.
FIG. 1 shows the activity on the medium when a Long MPDU with response expected is transmitted. The Response delimiter appears a prescribed duration—known as the Response Interframe Space (RIFS) duration—after the transmission of the Long MPDU. The Response delimiter is followed by a Contention Interframe Space (CIFS) and the priority resolution slots (PRS). Priority resolution slots are followed by contention slots.
FIG. 2 shows the activity on the medium when a Long MPDU with no response expected is transmitted. In this case, the Long MPDU is followed by a contention interframe space (CIFS) and the priority resolution slots (PRS), which are followed by contention slots.
The information contained in various delimiters (e.g., the Long MPDU and the Response) along with the knowledge of various interframe spacing is used by HomePlug 1.0 stations to determine the start time of the priority resolution slots.
HomePlug 1.0 stations process packets received from higher layers and transfer them to the desired destination(s) using Long MPDUs. If a packet cannot fit in a single Long MPDU, the packet is segmented and each segment is transmitted using a Long MPDUs. One important aspect of HomePlug 1.0 protocol is Segment Bursting. Segment Bursting allows a station to transmit MPDUs carrying segments of a packet in a single burst using a contention free access mechanism of HomePlug 1.0. The contention free access mechanism can also be to transmit multiple packets in a single burst.
The contention free access mechanism uses the CC and CAP priority information broadcast in the delimiters. Using this mechanism, a station that has multiple segments pending sets the CC to 0b1 and CAP to the packet priority in the first MPDU. For Long MPDUs that require a response, the receiver repeats the received CC and CAP in the Response delimiter. When stations detect a transmission with CC set to 0b1, they defer from priority resolution and subsequent contention if their traffic has a priority of less than or equal to CAP. Thus, the station continues to transmit the remainder of the MPDUs with CC set to 0b1. The last MPDU in the burst is transmitted with CC set to 0b0, thus allowing other stations to access the medium.
Although HomePlug 1.0 based systems are widely used, they are not ideal for supporting multimedia applications. This is due to the uncontrolled nature of channel access in HomePlug 1.0 networks makes it difficult to support quality of service (QoS) guarantees. QoS can be evaluated in terms of latency, jitter, and packet loss probability (PLP), where latency is how much time it takes for a packet of data to get from one device to another, jitter is a random variation in the delay between the arrival of one packet of data and a consecutive packet of data, and PLP is a probability of losing a packet of data transmitted from one device to another device. Other protocols that can support QoS over AC power lines can be hampered in the presence of HomePlug 1.0 legacy stations in the network. HomePlug 1.0 based systems also suffer from poor network performance resulting from segment bursting. Since segment bursting is common at low data rates, these connections use most of the time available. This unfair access can reduce-the network throughput.